Recently CRISPR-Cas9 system has been reported to be capable of targeting a viral RNA, and this phenomenon thus raises an interesting question of whether Cas9 can also influence translation of cellular mRNAs. Here, we show that both natural and catalytically dead Cas9 can repress mRNA translation of cellular genes, and that only the first 14 nt in the 5' end of sgRNA is essential for this process. CRISPR-Cas9 can suppress the protein expression of an unintended target gene without affecting its DNA sequence and causes unexpected phenotypic changes. Using the designed RNA aptamer-ligand complexes which physically obstruct translation machinery, we indicate that roadblock mechanism is responsible for this phenomenon. Our work suggests that studies on Cas9 should avoid the potential off-target effects by detecting the alteration of genes at both the DNA and protein levels.